FIELD OF THE INVENTION
-
The present invention is related to the formation of ferro-electric thin films. Particularly
PZT thin films are formed using a sol-gel technique. Such ferro-electric films can be
used as a dielectric in a ferro-electric capacitor or as a gate insulating layer in a ferro-electric
field effect transistor (FET). Both capacitor and FET can be applied in ferro-electric
memories for DRAM, FRAM and non-volatile applications.
BACKGROUND OF THE INVENTION
-
Ferro-electric thin films are used for the fabrication of ferro-electric non-volatile
memories. These memories can be stand alone memories as well as embedded
memories, e.g. embedded in a conventional CMOS process as used for the fabrication of
integrated circuits. Contrary to the conventional floating gate type non-volatile
memories, like e.g. flash or EEPROM, ferro-electric non-volatile memories do not need
high voltages, i.e. higher than the supply voltage which is defined by the choice of the
process wherein these ferro-electric memories are integrated. Consequently, switching
at voltages equal to or below the supply voltage has the advantage that no high-voltage
generators have to be integrated in said process. Currently, ferro-electric memories
using PZT (Lead-Zirconate-Titanate) films require switching voltages from 3 V to 5 V.
However due to the ongoing downscaling of the channellength of MOSFETs fabricated
in present and future deep submicron CMOS processes, the supply voltage will also
further scale down from 3.3V to 2.5V and to 1.5V. Furthermore, ferro-electric memories
are also interesting for portable applications due to their low power consumption. For
these portable applications, however, the supply voltage should be compatible with
single cell battery operation, i.e. operation has to be guaranteed even down to 0.9 V.
-
One of the methods to deposit a ferro-electric film, e.g. a PZT film, is the sol-gel
spin-on technique. Compared with conventional thin film forming processes such as
chemical vapour deposition, evaporation or sputtering, a sol-gel technique requires
considerably less equipment. Furthermore, throughout different wet chemical methods
for the preparation of ceramic oxides, the sol-gel technique offers great advantages
because the stoichiometry of the end product can easily be controlled and because it is
possible to obtain products for various applications, like e.g. bulk ceramics, fibres, thick
coatings or thin films, starting from almost the same precursor solution. In the sol-gel
technique, a chemical precursor solution is spin-coated on a substrate, afterwards
different thermal treatments are performed for the evaporation of the solvent, the
pyrolysis and the crystallization in order to form a ceramic oxide. Particularly, in case a
PZT precursor solution is used, a ferro-electric PZT film can be formed. When one
wants to produce a high quality ferro-electric film by means of a sol-gel technique, the
first requirement is of course a high quality precursor solution, whose characteristics
are adapted to the thin film fabrication.
-
Contrary to SBT (SrBi
2Ta
2O
9) material, where switching at 1.5V is already
demonstrated, the fabrication of PZT films on Si substrates showing fully saturated
switching at low voltage, i.e. below 3 V, is not yet demonstrated. The most
straightforward way of reducing the switching voltage is to decrease the film thickness,
provided that this does not deteriorate the electrical characteristics of the films,
particularly the value of the remnant polarization P
r and the rectangularity of the
hysteresis loops. The remnant polarization value P
r determines the signal magnitude for
a given capacitor area, and hence defines the minimal capacitor size. On the other hand,
a less rectangular or more slanted loop will result in higher switching voltages V
s which
is defined as the amplitude needed to obtain full hysteresis loop opening. However
published reports, like K.R. Udayakumar et al., J.Appl.Phys. 77(8), April 1995, pp. 3981-3986;
K. Amanuma et al., Jpn.J.Appl.Phys., Vol.32 (1993), pp. 4150-4153; L. E. Sanchez et
al., Appl. Phys. Lett. 56 (24), June 1990, pp. 2399-2401, on PZT film thickness scaling
effects indicate that the electrical characteristics of these films deteriorate if the
thickness decreases. Particularly, films which are deposited on Si substrates and use Pt
electrodes and which have a reduced film thickness, typically below 200 nm, show a
degradation of the electrical characteristics. In particular, one has observed:
- a decrease of the remnant polarization, Pr;
- a slanting of the hysteresis loop, i.e. the hysteresis loop becomes less rectangular;
- a decrease of breakdown field;
- an increase of the coercive field, Ec.
-
-
Consequently, the difference between coercive field and saturation field increases and
therefore the magnitude of the saturation field, i.e. the electrical field required to obtain
full saturated switching increases even stronger than the magnitude of the coercive field
with increasing thickness. Moreover, the minimum film thickness below which the
electrical characteristics start to deteriorate, strongly varies with the experimental
conditions. This indicates that the observed degradations are not intrinsic but due to the
ferro-electric capacitor fabrication process and particularly the process conditions for
the formation of a ferro-electric thin film and even more particularly the deposition
technique. It is therefore at present not clear, particular when said deposition technique
is a sol-gel technique, how to fabricate high quality thickness scaled ferro-electric thin
films, particularly for a thickness below 200 nm and more particularly for a thickness
below 100 nm.
SUMMARY OF THE INVENTION
-
In an aspect of the invention a method is disclosed for the formation of ferro-electric
films using a multi coating process based on a sol-gel technique. In particular
PZT films are formed using a sol-gel technique of an alkoxide-type liquid chemical PZT
precursor. The mass fractal dimension of the polymeric structures, where said solution
is composed of, has to be sufficiently low, i.e typically smaller than about 2.
Furthermore, hydrolysis steps, used during the preparation of said precursor solution,
have to be properly controlled to thereby avoid an increase of the mass fractal
dimension of the polymeric structures involved and assuring molecular homogeneity
throughout the entire film formation process. Preferably said precursor solution is a
Pb(Zr
xi
1-x)O
3 precursor solution which is prepared, according to an embodiment of the
invention, by means of an organic sol-gel technique. Preferably x is in the range from
0.1 to 0.4. Preferably said precursor is diluted in BuEtOH (2-butoxyethanol). Using a
more diluted precursor results in thinner coated layers, so in fact the thickness is
dependent upon the dilution. According to the present invention, a method for the
formation of a ferro-electric film on a substrate is disclosed, comprising the steps of:
- a) depositing a ferro-electric layer of a diluted chemical PZT precursor solution on
said substrate by means of a sol-gel;
- b) evaporating at least the majority of the solvents in said layer by means of a
thermal treatment;
- c) pyrolysis; and
wherein the sequence of steps a), b) and c) is performed at least twice and thereafter a
crystallization step is performed resulting in a film having a remnant polarisation
varying less then 30 % in said thickness range. Steps a), b) and c) can be performed
repeatedly until at least 2 layers and maximum 6 layers are deposited. Finally a
crystallization step is performed at temperatures typically at 700 °C or below.
Particularly said crystallisation step can be a heating step on a hot plate. Alternatively
said crystallization step can be a rapid thermal processing step in a furnace. In a
preferred embodiment of this aspect, no intermediate crystallization steps are
performed. The ferro-electric films formed using this method have excellent electrical
characteristics.-
-
In an aspect of the invention a method is disclosed for the formation of ferro-electric
films using a multi coating process based on a sol-gel technique. In particular
PZT films of an alkoxide-type liquid chemical PZT precursor are formed using a sol-gel
technique. Therefore the mass fractal dimension of the polymeric structures, where said
solution is composed of, has to be sufficiently low, i.e. typically smaller then about 2.
Furthermore, hydrolysis steps, used during the preparation of said precursor solution,
have to be properly controlled to thereby avoid an increase of the mass fractal
dimension of the polymeric structures involved and assuring molecular homogeneity
throughout the entire film formation process. Preferably said precursor solution is a
Pb(Zr
xTi
1-x)O
3 precursor solution which is prepared, according to an embodiment of the
invention, by means of an organic sol-gel technique. Preferably x is in the range from
0.1 to 0.4. Said precursor can be diluted in BuEtOH. According to the present invention
a method for the formation of a ferro-electric film on a substrate is disclosed,
comprising the steps of:
- a) depositing a ferro-electric layer of a chemical PZT precursor solution on said
substrate by means of a sol-gel technique;
- b) evaporating at least the majority of the solvents in said layer by means of a
thermal treatment;
- c) pyrolysis;
- d) crystallizing said layer at a temperature in the range of 700 °C or below 700 °C
and wherein steps a), b), c) and d) are performed at least twice thereby assuring that the
last step is a crystallization step and resulting in a film having a remnant polarisation
varying less then 30 % in said thickness range. Particularly said crystallisation step can
be a heating step on a hot plate. Alternatively said crystallisation step can be a rapid
thermal processing step in a furnace. Each time a layer is deposited, i.e. step a), the
evaporating (step b))and pyrolysis (step c)) steps are performed subsequently. After
each sequence of steps a), b) and c) one can choose to perform a crystallization step.
Preferably this crystallization step is performed if the thickness of the film formed since
the last crystallization step is minimum about 40 to 50 nm. Furthermore, at least two
layers are deposited thereby assuring that the last step is always a crystallization step.
The ferro-electric films formed using this method have excellent electrical
characteristics.-
-
In an aspect of the invention a PZT precursor solution is disclosed comprising
lead, titanium and zirconium. Particularly, a Pb(ZrxTi1-x)O3 precursor solution for ferro-electric
thin film fabrication is disclosed, said precursor solution comprising polymeric
structures having a mass fractal dimension equal to or smaller than 2 and having an
excellent molecular homogeneity, said precursor solution being diluted in
butoxyethanol and where x has a value in the range from 0.1 to 0.4. Starting products
are hydrate lead acetate with chemical formula PbAc2·3H2O, Zr(OPr)4 (70% in
propanol), Ti(OiPr)4 and 2-butoxyethanol. Before introduction in the PZT solution, the
hydrate lead acetate is dehydrated by means of recrystallization. To prepare the PZT
solution, said dehydrated lead acetate is mixed in a flask together with a solvent (2-butoxyethanol)
and Zr(OPr)4. The flask is connected to a condenser with a nitrogen
filled balloon on top. The mixture, containing a 16% (atomic) excess of said dehydrated
lead acetate, is flushed with the nitrogen and then heated in an oil bath up to 130°C for
six hours while agitating said mixture by means of magnetic stirring.
-
Further, the solution is cooled down and Ti(OiPr)4 is added. The system is again
flushed and refluxed for six hours at 130°C. After cooling down and weighing the filled
flask, the resulting clear solution is distilled under vacuum.
-
A solvent, preferably BuEtOH, is then re-added until the initial weight is reached,
and the solution is agitated long enough to be sure that the solvent is homogeneously
mixed with the solution. By using a mixture of 2 ml HNO3 (70%, Baker) and 50 ml of
BuEtOH, 0.0562 mole HNO3 per mole of PZT is added to the solution while vigorously
agitating.
-
After at least one hour 0.14 mole H2O per mole of PZT is added by using a
mixture of 1.02 ml deionised H2O and 50 ml of BuEtOH. Thus, both HNO3 and H2O are
mixed in an excess of a solvent before they are added to the PZT solution, resulting in a
homogeneous distribution of hydroxyl groups among the dissolved metal atoms,
thereby assuring molecular homogeneity throughout the entire film formation process.
BRIEF DESCRIPTION OF THE DRAWINGS
-
- Figure 1 represents a process flow diagram, according to an embodiment of the
invention, used for the preparation of a sol-gel precursor solution.
- Figure 2 represents a schematic cross-section of an examplary capacitor comprising a
dielectric layer, said dielectric layer being a ferro-electric film formed according to an
embodiment of the invention.
- Figure 3 represents a graph of the remnant polarisation of a film versus the voltage
applied on said film for different values of the switching voltage. Said film is a three-layer
ferro-electric film formed, according to an embodiment of the invention, without
any intermediate crystallization steps using a sol-gel technique of a Pb(ZrxTi1-x)O3
precursor solution where x=0.3. Said film has a thickness of about 180 nm.
- Figure 4 represents a graph of the remnant polarisation of a film versus the voltage
applied on said film for different values of the switching voltage. Said film is a three-layer
ferro-electric film formed, according to an embodiment of the invention, without
any intermediate crystallization steps, using a sol-gel technique of a diluted Pb(ZrxTi1-x)O3
precursor solution where x=0.3. Said film has a thickness of about 75 nm.
- Figure 5 represents a graph of the remnant polarisation of a film versus the signal
amplitude. Said film is a three-layer ferro-electric film formed, according to an
embodiment of the invention, without any intermediate crystallization steps, using a
sol-gel technique of a diluted Pb(ZrxTi1-x)O3 precursor solution where x=0.3. Said film
has a thickness of about 75 nm.
- Figure 6 represents a graph of the remnant polarisation of a ferro-electric film versus the
film thickness wherein said film is formed according to an embodiment of the present
invention, i.e. with intermediate crystallization steps, using a sol-gel technique of a
Pb(ZrxTi1-x)O3 precursor solution where x=0.2.
- Figure 7 represents a graph of the remnant polarisation of a ferro-electric film versus the
film thickness wherein said film is formed according to an embodiment of the present
invention, i.e. with intermediate crystallization steps, using a sol-gel technique of a
Pb(ZrxTi1-x)O3 precursor solution where x=0.3.
- Figure 8 represents a graph of the remnant polarisation of a ferro-electric film versus the
thickness per crystallization, i.e. the thickness of a part of the film formed since the
previous (intermediate) crystallization step untill the next (intermediate) crystallization
step is performed, wherein said film is formed according to an embodiment of the
present invention using a sol-gel technique of a Pb(ZrxTi1-x)O3 precursor solution where
x=0.2.
- Figure 9 represents a graph of the remnant polarisation of a ferro-electric film versus the
thickness per crystallization, i.e. the thickness of a part of the film formed since the
previous (intermediate) crystallization step untill the next (intermediate) crystallization
step is performed, wherein said film is formed according to an embodiment of the
present invention using a sol-gel technique of a Pb(ZrxTi1-x)O3 precursor solution where
x=0.3.
- Figure 10 represents a graph of the orientation (111)/(100) peak intensity ratio of a
ferro-electric film versus the thickness per crystallization, i.e. the thickness of a part of
the film formed since the previous (intermediate) crystallization step untill the next
(intermediate) crystallization step is performed, wherein said film is formed according
to an embodiment of the present invention using a sol-gel technique of a Pb(ZrxTi1-x)O3
precursor solution where x=0.2.
- Figure 11 represents a graph of the remnant polarisation of a film versus the voltage
applied on said film for different values of the switching voltage. Said film is formed
with a thickness per crystallization, i.e. the thickness of a part of the film formed since
the previous (intermediate) crystallization step untill the next (intermediate)
crystallization step is performed, below 40 nm.
- Figure 12 represents a graph of the remnant polarisation of a ferro-electric film versus
the voltage applied on said film for different values of the switching voltage. Said film
is a two-layer ferro-electric film formed, according to an embodiment of the invention,
with one intermediate crystallization step, using a sol-gel technique of a Pb(ZrxTi1-x)O3
precursor solution where x=0.3. Said film has a thickness of about 80 nm.
-
DETAILED DESCRIPTION OF THE INVENTION
-
A fundamental and straightforward prerequisite for the production of thin high
quality ferro-electric films using a sol-gel technique is a high quality precursor solution,
whose characteristics are adapted to the needs of thin film fabrication. The extend of
polymer branching or aggregation and the condensation rate are believed to determine
the packing efficiency and thus the quality of a thin ferro-electric film. Polymeric
species are structurally characterized by the mass fractal dimension, which is in fact a
measure for the extend of polymer branching. The mass fractal dimension influences
the steric constraints between separate polymeric structures. When the mass fractal
dimension has a high value, the polymers have elaborated branch structures, which
enhance cross-linking and therefore hamper the interpenetration of said polymers. As a
result a low packing efficiency is obtained leading to a high defect density of the
resulting film. On the other hand, a low value for the mass fractal dimension, leads to
weakly branched extended polymers, thereby reducing the possibility for cross-linking
and thus enhancing interpenetration of said polymers, and results in a high packing
efficiency and high quality films. Consequently, in realistic systems, polymeric
structures which are forced into close proximity of each other, e.g. by increasing the
concentration, can freely interpenetrate one another if the mass fractal dimension is
sufficiently low, i.e. typically smaller then about 2.
-
In general, hydrolysis, when not properly controlled, is believed to result in an
increase of the mass fractal dimension of the polymeric structures involved. Contrary,
acid-catalyzed hydrolysis conditions, especially with a low H2O/metal ratio and a
precise homogeneity control, are believed to result in weakly branched, extended
polymers. These polymers can interpenetrate during film formation, when the
concentration is increased by evaporation of the precursor solution, as long as
condensation does not inhibit the flow of said polymers. As a result, very dense high
quality films are formed.
-
In an embodiment of the invention a PZT precursor solution for ferro-electric thin
film fabrication is disclosed comprising lead, titanium and zirconium. Particularly a
1.12 molar Pb(ZrxTi1-x)O3 precursor solution (14) is prepared by means of an organic sol-gel
route (Figure 1). Preferably x is in the range from 0.1 to 0.4. Starting products are
hydrate lead acetate with chemical formula PbAc2·3H2O (commercially available from
Merck), Zr(OPr)4 (70% in propanol, commercially available from Fluka), Ti(OiPr)4 and 2-butoxyethanol
(both commercially available from Acros).
-
According to the present invention, before introduction in the PZT solution, the
hydrate lead acetate is dehydrated by means of recrystallization. This dehydration is
performed to prevent spontaneous uncontrolled hydrolysis. As mentioned before,
hydrolysis can lead to an increase of the polymeric mass fractal dimension. A preferred
way of performing said dehydration is described in the sequel.
-
The hydrate lead acetate is dissolved in butoxyethanol (BuEtOH) (1) at 60°C. The
resulting solution is kept at this temperature for two hours, and is cooled down slowly
(2) to thereby form a precipitate of dehydrated lead acetate. The precipitation yield can
be promoted by further cooling down the vessel containing said solution in an ice bath.
-
Said dehydrated lead acetate is then vacuum filtrated, twice washed with ethanol
(3) (commercially available from U.C.B.) and once with diethylether (U.C.B.).
-
Said dehydrated lead acetate is dried in a conventional furnace at 60°C (4) to
remove solvent remnants. After crushing the dehydrated lead acetate, it can be stored
for several months in a desiccator without any take-up of water. Unlike the hydrated
lead acetate, said dehydrated lead acetate is not soluble in butoxyethanol. However, in
the presence of at least one alkoxide it dissolves completely.
-
To prepare the PZT solution, said dehydrated lead acetate is mixed in a flask
together with a solvent (2-butoxyethanol) and Zr(OPr)4 (5) The flask is connected to a
condenser with a nitrogen filled balloon on top. The mixture, containing a 16% (atomic)
excess of said dehydrated lead acetate, is flushed with the nitrogen and then heated in
an oil bath up to 130°C for six hours (5) while agitating said mixture by means of
magnetic stirring.
-
Further, the solution is cooled down and Ti(OiPr)4 is added (6). The system is
again flushed and refluxed for six hours at 130°C (8). After cooling down and weighing
the filled flask, the resulting clear solution, said clarity being a good indicator for the
homogeneity of the solution, is distilled under vacuum to remove the esters that are
formed during reaction of the acetate with the alkoxides. Distilling said esters (9) leads
to a controllable hydrolysis.
-
A solvent, preferably BuEtOH, is then re-added (10) until the initial weight is
reached, and the solution is agitated long enough to be sure that the solvent is
homogeneously mixed with the solution. By using a mixture (11) of 2 ml HNO3 (70%,
Baker) and 50 ml of BuEtOH, 0.0562 mole HNO3 per mole of PZT is added to the
solution while vigorously agitating (12).
-
After at least one hour 0.14 mole H2O per mole of PZT is added (13) by using a
mixture of 1.02 ml deionised H2O and 50 ml of BuEtOH. Remark that this hydrolysis is
acid catalysed, which limits the condensation rate, and consequently has a positive
influence on the packing efficiency. Further remark that the hydrolysis ratio (mole
H2O/mole PZT) is 0.14, which is very low and thus also leads to an increased packing
efficiency.
-
Besides the ones already mentioned, there are two further crucial steps during the
above described PZT solution formation process having a major impact on the
properties of the ferro-electric thin films formed by means of a sol-gel technique using
said PZT solution. Both HNO3 and H2O are mixed in an excess of a solvent before they
are added to the PZT solution. When adding directly, part of the alkoxides can fully be
hydrolysed locally, thereby preventing a homogeneous distribution of hydroxyl groups
among the metal molecules. Furthermore, unlike most publications, like e.g. the
European patent EP 0674019 A1, butoxyethanol is used as a solvent in stead of
methoxyethanol. Besides the less toxic properties, it offers advantages towards film
density and concentration: As clearly seen in TG-FTIR measurements, condensation
proceeds at much higher temperatures in comparison to methoxyethanol based
solutions. This means that the interpenetration time is longer, resulting in a more
perfect film microstructure.
-
In an aspect of the invention a method for the formation of ferro-electric films by
means of a sol-gel technique is disclosed. The simplest way to form a ferro-electric film
is by using a so-called single coating process. As a result a ferro-electric film, composed
of a single layer, is formed on a substrate. Such a single coating process can comprise
the steps of:
- depositing a ferro-electric layer on said substrate by means of a sol-gel technique,
i.e. the spin coating of a chemical precursor solution on a substrate;
- evaporating at least the majority of the solvents by means of at least one thermal
treatment;
- pyrolysis, i.e. burning off of at least the majority of the organic compounds; and
crystallization.
However, particularly for the purpose of forming ferro-electric thick films with good
electrical characteristics, i.e. films having a thickness above about 100 nm, a so-called
multi coating process is used. Such a method is in fact a sequence of single coating
processes wherein the intermediate crystallization steps can be omitted. A reason
therefore is that after a ferro-electric layer is spin coated on a substrate, different
thermal treatments are applied. During the thermal treatment of the coated layer,
shrinkage occurs, e.g. due to the removal of solvent in the evaporation step and/or the
removal of organic material during the pyrolysis, resulting in tensile stress build-up.
This limits the maximum thickness of a ferro-electric layer formed by a single coating
process because cracking will occur if the layer is too thick. In general, attempts to
fabricate ferro-electric capacitors with good electrical characteristics using a single
coating process, even below the maximum thickness obtainable, were not successful
due to excessive leakage or shorting problems. This can be attributed to surface
asperities of the underlying bottom electrodes of said capacitors, which were either not
completely covered or not uniformly covered by the coated layer, and/or to particles
incorporated in the coated layer. Consequently, a single coating process is not suited for
forming high quality ferro-electric films.-
-
According to the present invention a method is disclosed for the formation of a
ferro-electric film on a substrate using a multi coating process based on a sol-gel
technique. Said substrate can be a GaAs or a Si wafer with thereon an electrode of a
conductive metal oxide, e.g. PtO
2, RuO
2 or IrO
2. In particular PZT films are formed
using a sol-gel technique of an alkoxide-type liquid chemical PZT precursor. The extend
of polymer branching and the condensation rate are believed to determine the packing
efficiency and thus the quality of a ferro-electric thin film. Therefore the mass fractal
dimension of the polymeric structures, where said solution is composed of, has to be
sufficiently low, i.e. typically smaller then about 2. Furthermore, hydrolysis steps, used
during the preparation of said precursor solution, has to be properly controlled to
thereby avoid an increase of the mass fractal dimension of the polymeric structures
involved. Preferably said precursor solution is a Pb(Zr
xTi
1-x)O
3 precursor solution which
is prepared, according to an embodiment of the invention, by means of an organic sol-gel
technique. Preferably x is in the range from 0.1 to 0.4. Preferably said precursor is
diluted in BuEtOH (2-butoxyethanol, which is commercially available from Acros).
Using a more diluted precursor results in thinner coated layers, so in fact the thickness
is dependent upon the dilution. According to the present invention, a method for the
formation of a ferro-electric film, with a thickness in the range from 50 nm to 350 nm, on
a substrate is disclosed, comprising the steps of:
- a) depositing a ferro-electric layer of a diluted chemical PZT precursor solution on
said substrate, by means of a sol-gel technique;
- b) evaporating at least the majority of the solvents in said layer by means of a
thermal treatment;
- c) pyrolysis; and
wherein the sequence of steps a), b) and c) is performed at least twice and thereafter a
crystallization step is performed resulting in a film having a remnant polarisation
varying less than 30% in said thickness range.
in said thickness range. Particularly, this method is a multi-coating process where
different coated layers are stacked and where heat treatments, being a sequence of
evaporation steps and pyrolysis steps, are applied after each coating step. By doing so, a
too large material shrinkage and tensile stress build-up during the final crystallization
step, i.e. after the last coated layer, is prevented. The number of coated layers depends
on the desired thickness of the ferro-electric film. Steps a), b) and c) can be performed
repeatedly until at least 2 layers and maximum 6 layers are deposited. Preferably,
especially if the desired fim thickness is 100 nm or below 100nm, the maximum
thickness of each layer is 50 nm or smaller than 50 nm. Finally a crystallization step is
performed at temperatures typically at 700°C or below. Particularly said crystallisation
step can be a heating step on a hot plate, e.g. at 600°C during 30 minutes in an air
ambient. Alternatively said crystallisation step can be a rapid thermal processing step in
a furnace, e.g. at 700°C during 5 seconds in an oxygen ambient. In a preferred
embodiment of this aspect, no intermediate crystallization steps are performed. A ferro-electric
film, formed without intermediate crystallization steps can comprise typically
up to 6 layers. In the latter case, only one final crystallization step has to be performed.
The resulting film has a very smooth, nearly indiscernible boundary structure between
the different coated layers. If a larger number of layers is required, intermediate
crystallization steps, e.g. every 3-5th layer, are required. The electrical characteristics of
the films, formed according to the aforementioned multi coating process, are not
dependent on the number of coated layers. A slight improvement of the electrical
properties, i.e. Pr value increases, with increasing film thickness is observed. This
improvement is only clearly observable for films with a thickness above 200nm. The
films having a thickness from about 50nm to 100nm can easily withstand 10V amplitude
(1kHz sine or triangular signals), i.e. corresponding with electrical fields of 2MV/cm,
which is better than thicker films, e.g. 170nm films can only withstand 14V amplitude
corresponding with an electrical field of 800kV/cm-
-
In an embodiment of the invention, as an example, a method is disclosed to
fabricate high-quality thickness scaled PZT films using a sol-gel technique of a
Pb(Zr
xTi
1-x)O
3 precursor solution, particularly a 1.12 molar Pb(Zr
xTi
1-x)O
3 precursor
solution which is prepared, according to an embodiment of the invention, by means of
an organic sol-gel technique (Figure 1) is used. In fact x equals 0.3 in this example. The
precursor solution is diluted according to a 1/2 ratio in BuEtOH. The aim of the
experiment (see also figure 2) was to form a ferro-electric film (25) on a Pt electrode (24),
said electrode being formed on a substrate, i.e. a silicon wafer (21) with a stack of a
silicon oxide layer (22) and a Ti layer (23) thereon. In fact this electrode can form the
bottom electrode, while the ferro-electric film can form the dielectric of a ferro-electric
capacitor, said capacitor further comprising a Pt top electrode (26). Furthermore the
final film thickness should be in the range from 170 nm to 200 nm and should have a
quality allowing the use of switching voltages in the range from 2.5 to 3V. For the
formation of the ferro-electric thin film, a multi coating process is used, comprising the
steps of:
- a) depositing a ferro-electric layer on a Pt electrode, formed on a substrate, by
means of a sol-gel technique, i.e. the spin coating of a chemical precursor solution,
particularly a spin rate of 3000 rpm during 30 seconds is used;
- b) evaporating at least the majority of the solvents in said layer by means of a
thermal treatment at 200 °C during 2 minutes on a hot plate in an air ambient;
- c) pyrolysis, i.e. burning off of at least the majority of the organic compounds, at 400
°C during 2 minutes on a hot plate in an air ambient;
The sequence of steps a), b) and c) is performed three times until three layers are
deposited. Finally a crystallization step is performed by means of a heating step on a
hot plate at 600°C during 30 minutes in an air ambient. The resulting film has excellent
electrical characteristics, i.e. high rectangularity loops and a high remnant polarisation
dependent of the switching voltage. In figure 3 the results for switching voltages of 1.5V
(31), 2V (32) and 2.5V (33) are presented yielding excellent characteristics for switching
voltages down to 2V. Remark that no intermediate crystallization steps are performed.
These intermediate crystallization steps are omitted because such a step results in a
clearly observable boundary layer at the interface between two subsequent layers in the
film. These boundary layers are believed to negatively influence the electrical
characteristics of the ferro-electric film. These intermediate crystallization steps can be
omitted if the total number of layers is not too large, because the largest amount of
material loss and consequently of the shrinkage occurs during the evaporation and
pyrolysis step.-
-
In a best mode embodiment of the invention, as an example, a method is disclosed
to fabricate high-quality thickness scaled PZT films using a sol-gel technique of a
Pb(Zr
xTi
1-x)O
3 precursor solution, particularly a 1.12 molar Pb(Zr
xTi
1-x)O
3 precursor
solution which is prepared, according to an embodiment of the invention, by means of
an organic sol-gel technique (Figure 1) is used. In fact x equals 0.3 in this example. The
precursor solution is diluted according to a 1/3 ratio in BuEtOH in order to form a
ferro-electric film with a thickness of 75 nm. For the formation of the ferro-electric thin
film a multi coating process is used, comprising the steps of:
- a) depositing a ferro-electric layer on an electrode, formed on a substrate, by means
of a sol-gel technique, i.e. the spin coating of a chemical precursor solution, particularly
a spin rate of 3000 rpm during 30 seconds is used;
- b) evaporating at least the majority of the solvents in said layer by means of a
thermal treatment at 200 °C during 2 minutes on a hot plate in an air ambient;
- c) pyrolysis, i.e. burning off of at least the majority of the organic compounds, at 400
°C during 2 minutes on a hot plate in an air ambient;
Step a), b) and c) are repeated until 3 layers are deposited.
Finally a crystallization step is performed at 600 °C during 30 minutes on a hot plate in
an air ambient. Remark that no intermediate crystallization steps are performed. The
ferro-electric film according to this example has excellent electric characteristics
- a high remnant polarization, Pr, typically about 30 µC/cm2 for a film thickness of 75
nm but also for films having a thickness in the range from 50 nm to 350 nm and for
switching voltages down to 1.5V (43) (see fig. 4 and 5 figure 4 further presents the
results for switching voltages of 0.5V (41), 1.0V (42) and 2V (44));
- a coercive field, Ec, typically about 100kV/cm for 75 nm films and about 80kV/cm
for films with a thickness of 150 nm or above;
- the hysteresis loop has excellent rectangularity, and the saturation voltage is 1.5V
(see also fig. 4 and 5);
- the film having a thickness from about 75 nm can easily withstand 10V amplitude
(1kHz sine or triangular signals), i.e. electrical fields of about 1.3 MV/cm;
- furthermore, no leakage is apparent form the hysteresis loops.
There are a number of properties, characteristic for the method of the present invention,
which result in said excellent electrical characteristics. At first, by using a multi coating
process, particle incorporation during a layer deposition will not lead to an uncomplete
coverage of the electrode. Further, if the surface of the electrode has a high degree of
roughness, again said multi coating process will lead to a better coverage compared
with a single coat process. Further, the precursor has been strongly diluted, resulting in
thinner coated layers. Thinner coated layers, having a thickness smaller than about 40
nm to 50 nm, have a higher material removal yield during the evaporation and
pyrolysis step. Consequently, films are obtained which have a denser structure and
have a better intrinsic quality resulting in a high value for the electrical breakdown
field. At last, because said multi coating process does not comprise intermediate
crystallization steps, there is no interlayer interface which can degrade or limit the
quality of the film. -
-
In an aspect of the invention a method is disclosed for the formation of a ferro-electric
film on a substrate using a multi coating process based on a sol-gel technique.
Said substrate can be a GaAs or a Si wafer with thereon an electrode of a conductive
metal oxide, e.g. PtO
2, RuO
2 or IrO
2. In particular PZT films are formed on a substrate
using a sol-gel technique of an alkoxide-type liquid chemical PZT precursor. Said
precursor solution should have characteristics which are adapted to the needs of thin
film fabrication. The extend of polymer branching and the condensation rate are
believed to determine the packing efficiency and thus the quality of a ferro-electric thin
film. Therefore the mass fractal dimension of the polymeric structures, where said
solution is composed of, has to be sufficiently low, i.e. typically smaller then about 2.
Furthermore, hydrolysis steps, used during the preparation of said precursor solution,
has to be properly controlled to thereby avoid an increase of the mass fractal dimension
of the polymeric structures involved. Preferably said precursor solution is a Pb(Zr
xTi
1-x)O
3
precursor solution which is prepared, according to an embodiment of the
invention, by means of an organic sol-gel technique. Preferably x is in the range from
0.1 to 0.4. Said precursor can be diluted in BuEtOH. According to the present invention
a method for the formation of a ferro-electric film, with a thickness in the range from 70
nm to 450 nm, on a substrate is disclosed, comprising the steps of:
- a) depositing a ferro-electric layer of a chemical PZT precursor solution on said
substrate, by means of a sol-gel technique;
- b) evaporating at least the majority of the solvents in said layer by means of a
thermal treatment;
- c) pyrolysis;
- d) crystallizing said layer at a temperature in the range of 700 °C or below 700 °C
and wherein steps a), b), c) and d) are performed at least twice thereby assuring that the
last step is a crystallization step and resulting in a film having a remnant polarisation
varying less then 30 % in said thickness range. Particularly said crystallisation step can
be a heating step on a hot plate. Alternatively said crystallisation step can be a rapid
thermal processing step in a furnace, e.g. at 700°C during 5 seconds in an oxygen
ambient.
-
-
So, according to the present invention, different coated layers are stacked and heat
treatments are applied after each coating step . Except for the last coated layer, i.e. the
intermediate layers, these heat treatments can be either a sequence of evaporation steps
and pyrolysis steps, or a sequence of evaporation steps, pyrolysis steps and
crystallization steps. Preferably these crystallization steps are only performed if the
thickness of the film formed since the last crystallization step is minimum about 40 to 50
nm. Concerning the last coated layer, said heat treatments is a sequence of evaporation
steps, pyrolysis steps and crystallization steps. Consequently, at least two layers are
deposited thereby assuring that the last step is always a crystallization step. By doing
so, a too large material shrinkage and tensile stress build-up during the final
crystallization steps, i.e. after the last coated layer, is prevented. The number of coated
layers depends on the desired thickness of the ferro-electric film. The electrical
characteristics of the films as formed are not dependent on the number of coated layers.
There are a number of properties, characteristic for the method of the present invention,
and resulting in said excellent electrical characteristics. In fact said method comprises a
multi coating process wherein a reduced number of coated layers is used but where
intermediate crystallization steps are used. The ferro-electric films formed using this
method have excellent electrical characteristics, i.e. a high remnant polarization, Pr,
typically about 30uC/cm2 for films having a thickness in the range from 70nm to
450nm, e.g. as it is presented in figure 6 using a Pb(ZrxTi1-x)O3 precursor solution with
x=0.2 and in figure 7 using a Pb(ZrxTi1-x)O3 precursor solution with x=0.3.
-
A reason why such excellent electrical properties are obtained despite the
intermediate crystallization is that this crystallization is performed at a moderate
temperature of 700°C or below. At this temperature, the crystallization is incomplete
resulting in a non-perovskite top-surface layer. This is caused by the loss of Pb (to the
atmosphere) and preferential Pb and Ti depletion in the uncrystallized part of the layer,
as e.g. in S. Wahl et al, Proceedings of Electroceramics V, Sept. 2-4, 1996, book I pp. 347-350.
Pb is known to be increasingly mobile with increasing temperature. Furthermore,
the crystallization rate of the ferro-electric, i.e. PZT, coated layer is enhanced when the
Pb and Ti content is increased but segregation of Pb and Ti takes place at the
crystallization interface. If however the Pb and Ti content becomes too low, complete
crystallization, i.e. perovskite phase conversion, stops. The minimum Pb and Ti content
required for complete crystallization is temperature dependent. If a subsequent coated
layer is applied on top of this layer, in the initial stage of a subsequent heat treatment,
e.g. an evaporation step, the lead and titanium content of the bottom layer is again
enriched from said subsequent layer. Afterwards, during the next crystallization step,
the crystallization of said subsequent coated layer can proceed without a strongly-deteriorated
barrier.
-
Another reason why such excellent electrical properties can be obtained despite
the intermediate crystallization steps is due to the fact that this intermediate
crystallization step is not performed until the thickness of a part of the film formed
since the previous crystallization step, hereafter refered to as the thickness per
crystallization, has a minimum value of about 40 to 50 nm, i.e. the so-called critical
thickness. Preferably this intermediate crystallization step is a rapid thermal processing
step in a furnace. It is observed that the remnant polarisation of the total film formed
deteriorates strongly if the thickness per crystallization has a value below the critical
thickness of about 40 to 50 nm, as e.g. presented in figure 8 where a Pb(ZrxTi1-x)O3
precursor solution with x=0.2 is used as well as in figure 9 where a Pb(ZrxTi1-x)O3
precursor solution with x=0.3 is used. It is also observed that the rectangularity of the
hysteresis loops of the total film formed deteriorates strongly if the thickness per
crystallization has a value below the critical thickness of about 40 to 50 nm, as e.g.
presented in figure 11, where a Pb(ZrxTi1-x)O3 precursor solution with x=0.2 is used and
where different switching voltages of 0.5V (111), 1V (112), 1.5V (113) and 2V (114) are
applied. X-ray diffraction measurements on PZT films with a thickness per
crystallization below the critical thickness reveal a loss of preferential (111) orientation
and an increase of the undesirable (100) orientation leading to the aforementioned
deterioration, as e.g. is presented in figure 10 where a Pb(ZrxTi1-x)O3 precursor solution
with x=0.2 is used for the film formation.
-
In a best mode embodiment of the invention, as an example, a method is disclosed
to fabricate high-quality thickness scaled PZT films using a sol-gel technique of a
Pb(Zr
xTi
1-x)O
3 precursor solution, particularly a 1.12 molar Pb(Zr
xTi
1-x)O
3 precursor
solution which is prepared, according to an embodiment of the invention, by means of
an organic sol-gel technique (Figure 1) is used. In fact x equals 0.3 in this example. The
precursor solution is diluted according to a 1/2 ratio in BuEtOH in order to form a
ferro-electric film with a thickness of 80 nm to 100 nm. For the formation of the ferro-electric
thin film a multi coating process is used, comprising the steps of:
- a) depositing a ferro-electric layer on an electrode, formed on a substrate, by means
of a sol-gel technique, i.e. the spin coating of a chemical precursor solution, particularly
a spin rate of 3000 rpm during 30 seconds is used;
- b) evaporating at least the majority of the solvents in said layer by means of a
thermal treatment at 200 °C during 2 minutes on a hot plate in an air ambient;
- c) pyrolysis, i.e. burning off of at least the majority of the organic compounds, at 400
°C during 2 minutes on a hot plate in an air ambient;
- d) crystallizing said layer by means of a heating step at 600°C during 30 minutes
on a hot plate in an air ambient. The sequence of steps a), b), c) and d) is performed
twice thereby forming a film comprising 2 layers. The ferro-electric film formed has a
thickness of 80 nm and has excellent electrical characteristics down to a switching
voltage of 2V: e.g. a high remnant polarization, Pr, of about 30 µC/cm2, as presented in
fig. 12 for different switching voltages of 1V (121), 2V (122), 3V (123) and 4V (124).
-